1. Field of the Invention
Embodiments disclosed herein relate generally to wellbore fluids. In particular, embodiments disclosed herein relate to aqueous based wellbore fluid that may find particular use in drilling a wellbore through a producing interval of the formation.
2. Background Art
During the drilling of a wellbore, various fluids are typically used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through wellbore to the surface. During this circulation, a drilling fluid may act to remove drill cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when circulation is interrupted, to control subsurface pressures, to maintain the integrity of the wellbore until the well section is cased and cemented, to isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, to cool and lubricate the drill string and bit, and/or to maximize penetration rate.
However, another wellbore fluid used in the wellbore following the drilling operation is a completion fluid. Completion fluids broadly refer any fluid pumped down a well after drilling operations have been completed, including fluids introduced during acidizing, perforating, fracturing, workover operations, etc. A drill-in fluid is a specific type of drilling fluid that is designed to drill and complete the reservoir section of a well in an open hole, i.e., the “producing” part of the formation. Such fluids are designed to balance the needs of the reservoir with drilling and completion processes. In particular, it is desirable to protect the formation from damage and fluid loss, and not impede future production. Most drill-in fluids contain several solid materials including viscosifiers, drill solids, and additives used as bridging agents to prevent lost circulation and as barite weighting material to control pressure formation.
During drilling, the filtercake builds up as an accumulation of varying sizes and types of particles. This filtercake must be removed during the initial state of production, either physically or chemically (i.e., via acids, oxidizers, and/or enzymes). The amount and type of drill solids affects the effectiveness of these clean up treatments. Also affecting the effectiveness of the clean up of the wellbore prior to production is the presence of polymeric additives, which may be resistant to degradation using conventional breakers.
Designing drill-in fluids which can guarantee minimum invasion into the reservoir rock is necessary for open hole completion wells. The industry has proposed several ideas to deal with the problem, most of them based on adding bridging agents to the fluid formulation. Such agents would block pores near the well bore and, consequently, prevent additional fluid to invade the rock.
Examples of formations in which problems often arise are highly permeable and/or poorly consolidated formation and thus a technique known as “under-reaming” may be employed. In conducting the under-reaming process, the wellbore is drilled to penetrate the hydrocarbon-bearing zone using conventional techniques. A casing generally is set in the wellbore to a point just above the hydrocarbon-bearing zone. The hydrocarbon-bearing zone then may be re-drilled to a wider diameter, for example, using an expandable under-reamer that increases the diameter of the wellbore. Under-reaming usually is performed using such special “clean” drilling fluids, drill-in fluids. Typically the drill-in fluids used in under-reaming are aqueous, dense brines that are viscosified with a gelling and/or cross-linked polymer to aid in the removal of formation cuttings. However, the expense of such fluids limits their general use in the drilling process.
When the target subterranean formation has a high permeability a significant quantity of the drilling fluid may be lost into the formation. Once the drilling fluid is lost into the formation, it becomes difficult to remove. Removal of the aqueous based well fluids is desired to maximize the production of the hydrocarbon in the formation. It is well known in the art that calcium- and zinc-bromide brines can form highly stable, acid insoluble compounds when reacted with the formation rock itself or with substances contained within the formation. These reactions often may substantially reduce the permeability of the formation to any subsequent out-flow of the desired hydrocarbons. As should be well known in the art, it is widely and generally accepted that the most effective way to prevent such damage to the formation is to limit fluid loss into the formation. Thus, providing effective fluid loss control is highly desirable to prevent damaging the hydrocarbon-bearing formation. For example such damage may occur during, completion, drilling, drill-in, displacement, hydraulic fracturing, work-over, packer fluid emplacement or maintenance, well treating, or testing operations.
One class of viscosifiers commonly used in the petroleum industry comprises polymeric structures starting with molecular weights of hundreds of thousands to several million grams per mole. These large, chemically bonded structures are often crosslinked to further increase molecular weight and effective viscosity per gram of polymer added to the fluid. Such types of viscosifiers include polymeric additives resistant to biodegration, extending the utility of the additives for the useful life of the mud. Specific examples of biodegradation resistant polymeric additives employed include biopolymers, such as xanthans (xanthan gum) and scleroglucan; various acrylic based polymers, such as polyacrylamides and other acrylamide based polymers; and cellulose derivatives, such as dialkylcarboxymethylcellulose, hydroxyethylcellulose and the sodium salt of carboxy-methylcellulose, guar gum, phosphomannans, scleroglucans, glucans, and dextrane.
Because of the high temperature, high shear (caused by the pumping and placement), high pressures, and low pH to which well fluids are exposed (“stress conditions”), the polymeric materials used to form fluid loss pills and to viscosify the well fluids tend to degrade rather quickly. In particular, for many of the cellulose and cellulose derivatives (such as HEC) used as viscosifiers and fluid control loss agents, significant degradation occurs at temperatures around 200° F. and higher. HEC, for example, is considered sufficiently stable to be used in an environment of no more than about 225° F. Likewise, because of the high temperature, high shear, high pressures, and low pH to which well fluids are exposed, xanthan gum is considered sufficiently stable to be used in an environment of no more than about 290 to 300° F. These large molecules are quite stable under the thermal conditions typically encountered in a subterranean reservoir. However, this thermal stability is believed to contribute to decreased well productivity. As a result, expensive and often corrosive breakers have been designed to destroy the molecular backbone of these polymeric structures. These breakers are typically oxidizers or enzymes and are at best only partially effective with typical reservoir cleanup less than 80% complete and more usually much less than 50% complete.
Accordingly, there exists a continuing need for wellbore fluids that are non-damaging to the formation and easily removed, particularly for use in drilling through a producing interval of a formation.